In a digital device, when a clock signal is defective or unstable, a system may malfunction. A bad connection of a portable terminal and distortion of data transmitted from a facsimile are mostly attributed to instability of a clock signal. The clock signal may be used as an operation source of a digital circuit serves as the core of the circuit. Therefore, frequency accuracy and stability are requirements of the clock signal. Their importance is increasing along with an increased demand for high-speed computation and fast processing. Moreover, the role of the clock signal is so significant to fields requiring reliability, such as communication equipment, that double or multiple clock signals are used for a normal operation in case one clock generation source has errors.